The invention relates to an improved process for the production of 2,6-di-tert-butylphenol with increased selectivity at low pressure and low temperatures by the reaction of 2-tert-butylphenol with isobutene under catalysis with aluminum tris-(2-tert-butylphenolate) in the presence of (1) saturated aliphatic and/or cycloaliphatic hydrocarbons and/or (2) excess liquid or dissolved isobutene and/or (3) C.sub.5 to C.sub.12 cycloalkenes, which contain no branchings in the C=C bond, and/or (4) alkenes of formula R.sub.1 --CH.dbd.CH.sub.2 (1-alkenes) or R.sub.2 --CH.dbd.CH--R.sub.3 with 5 to 16 C atoms.
It is known that 2,6-di-tert-butylphenol can be obtained by the addition of isobutene to phenol or to 2-tert-butylphenol in the presence of aluminum phenolates (cf. Ullmann, "Enzyklopaedie der Technischen Chemie [Encyclopedia of Industrial Chemistry]," Volume 18 (1979), page 205 ff.). The alkylation of phenol or 2-tert-butylphenol is usually performed in the presence of aluminum phenolate at 100.degree. C. and above, typically at 110.degree. C. to 120.degree. C., under pressure, which can be up to 25 bars. The catalyst is obtained by dissolving 1 to 3% by weight of aluminum in the phenol to be alkylated [cf. US-PS 2 831 898, DE-PSS 944 014, 1 044 825; J. Org. Chem. 22 (1957) 642; Ang. Chem. 69 (1957) 699]. It can also be produced by the reaction of aluminum alcoholates or of triethyl aluminum with phenol, as well as by other methods. The 2-tert-butylphenol or 2,6-di-tert-butylphenol obtained is a function of the ratio of the respective phenol used to the isobutene. Both 2,4-di-tert-butylphenol and 2,4,6-tri-tert-butylphenol occur as by-products. The use of excess isobutene results in high yields of the trisubstituted phenol [cf. Ullmann, "Enzyklopaedie der Technischen Chemie, " Volume 18 (1979), p. 200; Kirk-Othmer, "Encyclopedia of Chemical Technology," Vol. 2 (1978), p. 85)]. In addition, with increasing reaction time, there is an increase in the portion of compounds containing a para-position tert-butyl substituent (2,4-di-tert-butylphenol, 2,4,6-tri-tert-butylphenol). Therefore, the reaction is interrupted at an optimal moment for the production of 2,6-di-tert-butylphenol. At the end of the alkylation, the aluminum phenolate catalyst is deactivated, at the latest, before the separation of the reaction products by distillation to avoid dealkylation of the products. In general, this deactivation takes place by hydrolysis with water, acid or diluted lye, but even traces of acids have to be removed or neutralized again before the distillation. This requires (cf. Ullmann, p. 202) a careful washing of the alkylate with lye and water, and waste water results, which (after separating of the aqueous phase) has to be subsequently treated, to avoid polluting the environment. The rectification of the organic phase obtained according to this production process yields, for example, with the use of 2 mol of isobutene per mole of phenol, about 75% of theory of 2,6-di-tert-butylphenol and about 10% of theory of 2-tert-butylphenol. The distillation residue consists primarily of 2,4,6-tri-tert-butylphenol. Alternatively, if only 1 mol of isobutene per mole of phenol is used, then in addition to unreacted phenol and some 2,6-di-tert-butylphenol, 2-tert-butylphenol is isolated as the main product, which is obtained in about a 70% yield in relation to the reacted phenol.
If the catalyst is produced from aluminum organic compounds, preferably trialkyl aluminum, and 2-tert-butylphenol instead of the phenol, the addition of isobutene to 2-tert-butylphenol is successful even at temperatures below 100.degree. C. (cf. U.S. Pat. No. 3,355,504). Despite the lower reaction temperatures, the alkylation quickly produces the desired 2,6-di-tert-butylphenol, and with the use of comparable catalyst amounts (about 1-3 mol % in relation to the 2-tert-butylphenol used), this compound yields smaller portions of undesirable admixtures (such as 2,4-di-tert-butylphenol or above all, 2,4,6-tri-tert-butylphenol). Also, this special catalyst system must be deactivated before the beginning of the working up so as to avoid the dealkylation and realkylation during the separation of the reaction products by distillation.
Independent of the type of aluminum containing catalyst used, obtaining 2,6-di-tert-butylphenol by alkylation of phenol or 2-tert-butylphenol is always performed without using solvents according to the process of the prior art [cf. Ullmann, "Enzyklopaedie der Technischen Chemie," Volume 18 (1979), p. 202]. In this way, the isobutene is added so quickly as to allow the removal of heat from the reactor. This is necessary since the exact adherence to a narrow temperature range is a prerequisite for the production of 2,6-di-tert-butylphenol with the indicated yields (of about 75% of theory) in using the aluminum tris(phenolate) catalyst (cf. Ullmann, pp. 205/206). Obviously, this is mainly used. On the other hand, in the case of the alkylation of 2-tert-butylphenol in the presence of the aluminum tris-(2-tert-butylphenolate) catalyst, a considerable temperature increase is observed within a few minutes after the addition of isobutene [cf. U.S. Pat. No. 3,355,504, examples 2 and 3], which makes the production of 2,6-di-tert-butylphenol at a constant, low reaction temperature difficult and may make it impossible on a larger scale.
Disadvantages in the older processes of the prior art are in their need for relatively large amounts of the only moderately active aluminum tris-(phenolate) catalyst, which just as the aluminum tris-(2-tert-butylphenolate) catalyst, must be deactivated before the separation of the reaction products by distillation. The removal of the resulting waste water containing aluminum compounds and (alkyl-)phenols is absolutely necessary for reasons of environmental protection, but not without problems. This is indicated by the number of patent applications which deal with the deactivation of catalysts as well as with quite costly solutions to the disposal of waste water or with the reduction of the resulting amount (cf., e.g., U.S. Pat. No. 3,200,157, DE-PS 1 809 555, DE-OS 2 039 062, U.S. Pat. No. 3,939,215, DE-PS 2 602 149, BE-PS 842 691, U.S. Pat. No. 3,652,685, U.S. Pat. No. 3,970,708). The problems connected with the removal of waste water thus far could not be solved by heterogenization of known catalysts (cf. EP 206 085), since the catalytically effective aluminum phenolate catalysts are discharged with the reaction products to a significant extent.
Another drawback of the processes of the prior art consists in the formation of a high portion of undesirable di- and above all tri-alkylated products when using the aluminum tris-(phenolate) catalyst for the addition of isobutene to phenol. The separation and purification by distillation of the desired 2,6-di-tert-butylphenol are thus made considerably more difficult, and a significant portion of material used cannot easily be converted into usable products. The process of U.S. Pat. No. 3,355,504 makes it possible to considerably reduce the portion of 2,4,6-tri-tert-butylphenol and to produce 2,6-di-tert-butylphenol with yields of about 93% of theory. However, also in this process, the removal of the reaction heat and the adherence to a specified temperature in using the catalyst amounts indicated in examples 2 and 3, is similarly difficult as in the conventional process. In this process--as already mentioned--the isobutene feed optionally has to be matched to the heat removal to keep the reaction temperature in the required range, and finally, after an optimal 2,6-di-tert-butylphenol portion is reached, must be quickly and completely interrupted to prevent the formation of considerable portions of the undesirable 2,4,6-tri-tert-butylphenol. The production of 2,4,6-tri-tert-butylphenol influences the economic efficiency of the usual processes greatly so that its recycling into the alkylation process was recommended despite a considerable degree of technical complexity and only a modest increase of the 2,6-di-tert-butylphenol yields. (cf. U.S. Pat. No. 4,560,809). The process performed at about 100.degree. C., moreover, requires (1) technical devices for heating the reactor when the catalyst is formed (from aluminum) and at the beginning of the alkylation and (2) efficient cooling devices for temperature control during the reaction.